At every moment of every day, morning or midnight regardless of the season, there are birds aloft in the skies of the Western Hemisphere, migrating.

If it is spring or fall, the great pivot points of the year, then the continents are swarming with billions of traveling birds -- a flood so great even the most ignorant or unobservant notice the skeins of geese or the flocks of robins.

Bird migration is the one truly unifying natural phenomenon in the world, stitching the continents together in a way that even the great weather systems, which roar out of the poles but fizzle at the equator, fail to do. In Living on the Wind, a finalist for the 2000 Pulitzer Prize, Scott Weidensaul follows awesome kettles of hawks over the Mexican coastal plain, the bar-tailed godwits that hitchhike on gale winds 6,800 miles nonstop across the Pacific from Alaska to New Zealand, and the myriad songbirds whose numbers have declined so drastically in recent decades. Migration paths form an elaborate global web that shows serious signs of fraying, and Weidensaul delves into the tragedies of habitat degradation and deforestation with an urgency that brings life to the vast problems these miraculous migrants now face, while illuminating what may be the most compelling drama in the natural world.